Apparatus and method for laser welding of ribbons

ABSTRACT

An apparatus and method are provided for using laser energy in an automated bonding machine to effect laser welding of ribbons and other connectors, particularly conductive ribbons in microelectronic circuits. The apparatus and method allow bonding and connection of microelectronic circuits with discrete heating avoiding heat damage to peripheral microelectronic components. The apparatus and method also allow bonding of flexible materials and low-resistance materials, and are less dependant on substrate and terminal stability in comparison to existing bonding methods. The bonding method leads to decreased apparatus wear in comparison to existing bonding methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent is a divisional of U.S. patent application Ser. No.09/688,427 filed Oct. 16, 2000, now U.S. Pat. No. 6,501,043, and claimsthe benefit of U.S. Provisional Application No. 60/161,103, filed Oct.22, 1999.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for joiningconnective members, especially conductive members to join electronic andmicroelectronic circuits.

BACKGROUND

Electronic circuits, such as microelectronic logic circuits, or othersimilar electronic circuits, may be fabricated as an integrated unit,which has developed to a highly efficient method of compact circuitmanufacture. Ultimately, however, the integrated circuit components orother packaged electronics must be connected to larger circuits in orderto be utilized, and be interconnected via lead frames or otherconnectors with circuits such as input and display apparatus, powersupplies and grounds, and complementary circuits. Given the small scaleof such circuits, the connection of these circuits also takes place on arelatively small scale. For example, integrated circuit chips aretypically less than 0.3 inches×0.3 inches. These circuits may beinterconnected by very small wires, e.g. wires with a one mil diameter,or by small, flat conductive metal ribbons which may be, for example,1×10 mils (0.001 inches×0.01 inches).

Generally, metals for electronic connections may be joined by soldering,i.e., by the melting of an alloy or element with a relatively lowmelting temperature, where neither base material of the joined membersis melted or becomes part of the joint. Welding, in contrast, involvesthe melting of the base members to be joined, resulting in the formationof a weld nugget consisting of material from both of the elements to bejoined, in other words, a fusion or thorough and complete mixing betweenthe two edges of the base metal to be joined.

While heat is often used to join wires or other conductors together,both for solid-state, fusion, and solder/brazing applications, manytraditional methods of heating have proved to have drawbacks inmicroelectronic applications. One method of applying heat to a bondingsite has involved heating the bonding head to convey heat to the bondingsite. In an alternate method, a heater block may be clamped to a circuitlead frame. However, heat applied to these structures, which are largerelative to the area to be bonded, may cause distortion or bending ofthe lead frame, or damage to other electrical components. If heat is tobe used to solder microelectronic connectors, it would be desirable tomore precisely localize this heat on the leads or connectors to besoldered, rather than heating an area as large as, for example, anentire lead frame.

As an alternative to soldering using heat alone, ribbon bonders of theprior art have used specialized solid-state bonding methods, e.g.,ultrasonic energy, to bond the ribbons to substrates, lead frames, orvarious electronic components. Ultrasonic energy, a high-frequencyvibration, e.g. from 60 KHz to over 100 KHz, is imparted to the parts tobe bonded by a bond head. This vibration, and the attendant abrasion ofthe connector against the terminal pad or lead, in conjunction with heatand mechanical pressure from the bonding head, effects metallurgicalatomic diffusion bonding of the connector with the metal of the bondingsites. Modern ultrasonic bonding machines conveniently employ optics andpattern-matching logic systems in order to automate the bonding processfor a particular package or circuit being assembled.

Ultrasonic ribbon bonding is primarily employed as a method ofconnecting integrated circuits, packages or substrates in high frequencyor high power applications. Ultrasonic bonding techniques, however, haveseveral drawbacks which motivate a reduced reliance on such techniques.Ultrasonic or thermosonic bonding, i.e. ultrasonic vibration using heat,may find application in bonding flat, rigid structures but is not wellsuited to bonding less rigid, i.e., flexible or semi-flexiblestructures. Such structures tend to vibrate in response to theultrasonic energy causing much of the energy to be lost rather thancreating the intended bond. Another drawback of ultrasonic bonding isthat it may be used primarily with certain materials, and is generallylimited to gold, aluminum and copper. Accordingly, the substrate metalto be bonded is generally gold plated.

Because the ultrasonic energy is, in fact, a vibration, albeit a veryhigh-frequency one, this vibration may cause undesired movement of theparts to be joined during the bonding process. Not only can this lead todislocation of the parts vis-à-vis each other, but the movement of theparts during the time when bonding is being effected naturally resultsin a weaker and inconsistent bond. These problems present substantialvibrational stability requirements for the terminals and substrates usedin ultrasonic bonding. In light of the limitations of ultrasonicbonding, an alternative method of bonding conductors for microelectronicdevices would be desirable.

Resistance welding has enjoyed limited application in microelectronicsmanufacture. To varying extents, metallic objects resist the flow ofelectrical current. This resistance will cause heat energy as electriccurrent passes through the metals to be bonded. The higher the amperageand duration of current, the greater the heat energy that will beproduced. Metallic objects have thermal properties, a melting point, aspecific heat content, thermal conductivity, and more. By using theseproperties, an environment can be created to produce a molten pool thatwill harden into a welding nugget. However, the application ofresistance welding is limited, and is generally incompatible with lowresistance ribbon materials such as copper, silver and gold.

Laser-generated heat has found application in certain part-joiningmethods. For example, lead frames have been soldered with lasers inapplications such as TAB (Tape Automated Bonding), utilized, forexample, in U.S. Pat. No. 4,893,742 to Bullock. TAB bonding, however, issubject to a number of limitations, chief among them that dedicated andexpensive equipment is necessary for each process step. Theinterconnecting ribbons or leadframes must be formed ahead of time.Therefore, the specially designed tape carriers for each type of circuitbeing produced involve long lead time and high cost. Dedicated toolingis required to excise and form TAB leads. “Bumping”, i.e., the placementof small metal bumps on the circuit bond sites in order to provide abonding surface above the circuit's passivation layer, is required. Inaddition, the leadframes must be placed and held precisely in positionbefore soldering. Finally, the leads to be bonded with existingapparatus and methods require solder to effect bonding—the main body ofthe lead and connectors do not reach a melting point, and only thesolder is softened. TAB leads are accordingly coated with solder at somepoint prior to the bonding process. Therefore, considerable advancepreparation of the TAB leadframes is required. In general,characterizing the behavior of individual designs and structures is verytime-consuming, as is the construction of lead frame tapes for TABproduction methods.

It would be desirable to provide laser bonding which could be effectedusing devices similar to traditional wedge bonding equipment, which mayautomatically bond individual contact points, without the preparationrequired for tape-mounted lead frames. It would also be desirable toprovide a bonding method that would work on a variety of materials,including low-resistance metals, without the use of solder. In addition,it would be desirable to provide a bonding technique that utilizes ahighly localized heating area, without peripheral heating of lead framesor other components adjacent the bond site. Finally, it would bedesirable to have a bonding method capable of bonding flexible materialsthat may tend to vibrate in response to the application of ultrasonicenergy.

SUMMARY OF THE INVENTION

Difficulties with existing systems of connector fabrication are overcomewith a device providing for laser bonding of ribbon connectors,especially conductive connectors used to provide current pathways forthe operation of electronic or microelectronic components. In analternative embodiment, the device is adapted for use with any materialwhich absorbs the particular wavelength of the laser used. For example,the device may be used to bond non-conductive ribbons, i.e. plasticconnectors such as may be used in packaging or other applications. In afurther alternative embodiment, the invention may also be used to bondconductive connectors of alternative configurations, e.g. round wire.Preferably, a bonding device according to the present invention utilizesautomation as developed for traditional wire and ribbon bonding, e.g.pattern-matching automation.

According to one embodiment of the invention, an automatedpattern-matching bonder welds one end of an interconnection ribbon, theribbon being fed from a spool of suitable interconnection ribbon.Subsequently, the bond head of the device moves to the second bondlocation as it spools out and forms the ribbon into the desired loopshape and then welds the second connection and terminates the ribbon.Alternatively, after formation of a second bond, additional ribbon maybe spooled out to form one or more additional loop connectors, each loopterminating at a new weld connection. As an alternative to movement ofthe bond head, a machine table on which the work piece is mounted maymove while the bond head remains stationary. The present invention haspotential application for internal device interconnection, i.e.,connections internal to an IC package, as well as final board assemblyand other microelectronic connections. When bonding is effectedaccording to the present invention, the choices for both ribbon andsubstrate materials increases, and the dependence on structural rigidityand terminal stability, as required for ultrasonic bonding, decreases oris eliminated.

Microelectronic bonding has enjoyed particularly useful application inthe implantable medical device art, for example, as demonstrated in U.S.Pat. No. 5,535,097 to Ruben, et al. and U.S. Pat. No. 5,522,861 toSikorski, et al., both assigned to the assignee of the instantapplication and both of which are hereby incorporated by reference. Byway of example, the present invention may be used to make electricalconnectors between and among the hybrid circuit, battery, capacitors,feedthroughs, and other components of implantable medical devices. Inaddition, however, it will be appreciated to those skilled in the artthat the instant invention may be used in various microelectronicapplications. These may include, but are not limited to, semiconductorproduction and chip utilization, integrated circuit packaging andmounting, and other electrical interconnections in the computer hardwareand electronics industries.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a cross-section of part of a ribbon bonding apparatus inaccordance with a representative embodiment of the invention.

FIG. 2 is a plan view of an aspect of the ribbon bonding apparatus ofFIG. 1.

FIG. 3 is an alternate plan view of an aspect of the apparatus of FIG.2.

FIG. 4 is an alternate plan view of an aspect of the apparatus of FIG.2.

FIGS. 5-8 illustrate a method of bonding of a ribbon in accordance withan embodiment of the invention.

FIG. 9 is a cross-sectional view of a ribbon bonding apparatus accordingto the present invention.

DETAILED DESCRIPTION

FIG. 1 depicts a cross-section of a portion of a ribbon bondingapparatus in accordance with a representative embodiment of theinvention, the portion being a bond head 100. A ribbon 102, such asconductive metal ribbon, is supplied on a standard spool, depicted in alater figure. The bond head 100 is adapted to receive ribbon 102 througha threading slot 104. Threading slot 104 has a ribbon entrance 104 a andribbon exit 104 b. The slot 104 is preferably adapted to admit ribbonsof various thicknesses, e.g., 1 mil. From threading slot 104, ribbon 102is disposed under bond foot 106, and may be welded to bond substrate 108by a laser beam. In embodiments of the subject invention in which thelaser is not directed by optic fiber to bond site through bond head 100,the bond head 100 may be adapted to have a laser aperture 110 to admitlaser light that portion of ribbon 102 disposed over bond site 112. Theribbon may initially be threaded through threading slot 104 in bond head100, to ensure proper placement of ribbon 102 relative to bond head 100.As described herein, the bond head descends to the bond surface andforces the ribbon to contact the substrate or component to be bonded,e.g., an IC bond pad. Once the predetermined load is applied, a lasermay be fired in order to weld the ribbon 102 to the substrate 108 orother component.

Bond foot 106, as depicted in FIG. 1, is disposed at the bottom of bondfoot 100, which is shown in plan view in FIG. 2. As shown in FIG. 2,laser aperture 110 is disposed approximately in the center of bond foot106, which turns out from elongate bond head body 114. These features ofbond head 100 are shown in greater detail in plan view in FIG. 3. Theunderside of bond head 100 and bond foot 106 is shown in FIG. 4. As canbe seen in FIG. 4, laser aperture 110 is preferably substantiallycircular in shape, although other shapes suitable for passage of laserenergy are possible. Ribbon threading slot 104 has threading slotentrance 104 a, into which ribbon can enter from the back of bond head100. Threading slot 104 also has threading slot exit 104 b, from whichribbon may exit while ribbon is being spooled out in connection withloop formation, described herein.

A process of connection loop formation according to an embodiment of thepresent invention is depicted in FIGS. 5-8. In a preferred embodiment ofthe invention, the bond head has several axes or modes of travel, forexample, along an x-axis, y-axis, z-axis (vertically), and theta(rotation). By these various modes of travel as shown in FIG. 5, thebond head 106 may be positioned at a first bond site 112 upon bondsurface 116. The bond head will first descend along the z-axis in orderto contact the ribbon 102 to bond surface 116. Ribbon 102 is thereforedisposed between bond foot 106 and bond surface 116, and thereby held inplace. The ribbon 102 is fed through the bond tool ribbon threading slot104 which holds ribbon 102 in place during bonding. Ribbon 102 mayfreely pass through ribbon threading slot 104 while bond head 100travels between first and second or between subsequent bonds or welds.Upon placement of bond head 110 at a first bond site 112, apredetermined force is applied towards the bond site 112 by the bondhead 110. In one embodiment, the bond head is adapted to receive anoptic fiber through its length which allows the laser to fire directlyat the point of the weld as depicted in later figures. A suitable laserbeam is then fired, for example, through the aperture 110 on the bondfoot 106, heating a portion of the ribbon 102 as well as the bondsurface 116 above their respective solidus temperatures. A weld nugget,not depicted, is thus formed at the bond site 112, the nugget consistingof previously molten material of both the ribbon 102 and bond surface116. The weld nugget created by the laser firing may be expected to forma bond of greater strength and reliability than solder or braze bonds.

As shown in FIG. 6, following the initial bond as described above, orwhile the weld nugget of the initial bond is cooling, the bond head 100may move to a second bond site 120, such as a programmed site, throughan automated or otherwise predetermined trajectory adapted to spool outfrom bond head 100 a desired length of ribbon 102 to form loop 118. Asshown in FIG. 7, upon contact with a second bond site 120, ribbon 102 isagain disposed between bond foot 106 and bond surface 122, with a laserfiring as before through laser aperture 110 to form a weld nugget at thebond site. Thereafter, further ribbon 102 may be spooled out from ribbonspool in order to form a connected second loop from a continuous lengthof ribbon. Alternatively, as shown in FIG. 8, ribbon 102 may beterminated by clamping of ribbon 102 above bond head 100 or by lockingof the ribbon spool, not depicted. Following clamping of ribbon 102,bond head 100 is preferably moved in a manner leading to breaking ofribbon 102 in the vicinity of second bond site 120. In essence, ribbon102 is terminated by head, table or clamp motions as is typical ofexisting methods of ultrasonic ribbon bonding. In a preferredembodiment, additional ribbon 102 is played out from a ribbon spool inorder to be disposed under bond foot 106 for reinitiation of the bondingprocess as described above.

In an alternate embodiment of the invention, “security welds”, i.e.,double or other multiple welds may be effected at each bond site. Thesesecurity welds serve to increase contact area for improved current flow,mechanical strength, and reliability. The bonder makes the weld, movesslightly and welds the ribbon again to the same terminal. The welds mayoverlap, may combine to form a single uniform weld nugget, or may becompletely separate effecting discrete weld nuggets.

While typically described herein and with reference to FIGS. 5-8, as amovement of the bond head from a first bond site to a second bond site,as provided in a preferred embodiment, this bond head motionalternatively may be a relative motion only with regard to the workpiece containing bond sites, a work table, or the like. In other words,what is generally termed the bond head motion may be one of or acombination of head, table or work piece movements vis-à-vis each other.

In a representative embodiment of the subject invention and as depictedin FIG. 9, a laser beam may be directed at the bond site without passingthrough the entire length of bond head 110, thus allowing greaterflexibility in motion of the bond head, particularly with regard to 2(vertical) and theta mode movement (rotation). FIG. 9 depicts a crosssection of a portion of a laser bonding apparatus 130 according to thepresent invention. As illustrated in FIG. 9, a preferred embodiment ofthe subject invention utilizes a laser, the beam 140 of which isdelivered to the bond site 112 through a glass optic fiber 132 to aseparate focusing head containing lenses and optics suitable forfocusing the laser to bond site 112; thereby according full x, y, z, andtheta mode movement to bond head 100 without attendant stress on a glassoptic fiber 132 that would otherwise extend to bond foot 106 or bondtool 100.

It is believed that methods of laser delivery other than optic fiberrunning all the way to the bond head may reduce the possibility ofcertain occurrences such as fiber optic breakage, crimping orsplintering of optic fiber 132. For example, as shown in FIG. 9, anoptic fiber 132 may be directed towards the bond site 124, the opticfiber 132 being located within a separate arm or sheath 136. Becauseoptic fiber 132 does not extend all the way to bond head 110, but onlyto optical fiber/laser housing connector 134, bond head 110 may be movedaccording to z (vertical) or theta (rotation) modes without resultingundue stress on optic fiber 132. Connector 134 joins optic fiber sheath136 with laser path housing 138. The laser light beam 140 may bedirected at the bond site 124 in a way in which the laser beam isunfocused and consists substantially of light beams traveling parallelto each other. In an alternate embodiment, the laser may be focused, forexample, by the use of a lens. In this embodiment of the invention, thefocus point 142 of the laser beam 140 may be placed directly at oradjacent the bond site 112, for example, at the top surface of theribbon, at the bond surface 116 or slightly below the bond surface 116.

Optic fiber 132 may introduce laser beam 140 to collimating lens 142,having the effect of spreading out the laser beam into a broader beam.Collimated beam 144 then proceeds to dichroic mirror 146, where the beamis directed 90 degrees downward towards bond site 124. Broad beam 144may be focused by focusing lens 148, in order to supply a laser focuspoint 142 at the surface of or at some depth of bond site 124 accordingto the desired heating effect.

By use of dichroic mirror 146, broad collimated laser beam 140 may bereflected toward bond site 124. However, camera 150 or other visualsighting accommodation such as an eyepiece for direct viewing may beadapted at top of housing tube 152, in order to afford viewing forprocess monitoring, and if desired, other user operations such as manualalignment of focused laser beam 142 through laser aperture 110 and/ormanual placement of bond head 100 to desired bond site, e.g., bond site112.

In a preferred embodiment of the subject invention, bond head 110 andbond head arm 154 may be moved within certain modes of movement, such asx, y, z and theta (rotation) modes, without corresponding or attendantmovement of laser housing 138. Primarily, for example, user may wish toeffect movement of bond head 100 and bond arm 154 within the z mode(horizontal or “up and down” movement of the bond head), or theta mode(rotation of bond head 100 relative to bond site 112 or work piece 156without moving laser housing 138. In one embodiment of the subjectinvention, while z mode and theta mode bond head movement is independentof movement of laser housing 138, movement of bond head 100 according tox and y modes (i.e., planar movement of the bond head along the plane ofthe work piece) corresponds to an equal movement along the respective xand y mode of laser housing 138. In other words, in one embodiment laserhousing 138 moves in concert with bond head 100 and bond arm 154 inorder to maintain vertical alignment of laser focus point 142 with laseraperture 110 of bond head 100.

In an embodiment of the present invention in which laser housing 138 maybe moved independently from bond arm 154, preferably z mode verticalmovement of bond head 100 away from bond site 112 will generally beeffected only at times when laser beam 140 is not firing. These timesgenerally will be during loop formation, i.e., during the period whenribbon clamp 158 is open, allowing ribbon 102 to feed from spool 160while ribbon 102 is anchored to a first bond site such as bond site 112of FIG. 6 and being thus anchored is drawn from spool 160 due to bondhead 100 movement away from first bond site 124, with subsequenttermination of bond loop 118 upon bonding of second bond site 120 asdepicted in FIG. 7. In one embodiment of the present invention, an inertcover gas, e.g. helium, argon, or nitrogen is disposed through gasnozzle 162 by jetting over the bond site in order to prevent oxidationor discoloration of bonded ribbon 102, bond site 112 or the weld nugget.Inert gas jet from gas nozzle 162 also serves to keep lens 148 free ofvapors and debris.

The bond head 100 may be configured in alternate ways in order todeliver laser beam energy to the bond site. For example, optic fiber 132may extend directly through bond head 100 and bond foot 106 to bond site112. The optic fiber 132 may be completely embedded within bond head100, Alternatively, the optic fiber 132 may be free of bond head 100until entering bond foot 106 in a position corresponding to laseraperture 110 of bond foot 106.

In any of various embodiments of the subject invention, a Nd:YAG355-1,064 nm laser may be used. Generally, however, the laser source toeffect the bonding may be a high power pulsed or continuous wave (CW)laser, e.g. NdYAG, Ar-ion, Carbon dioxide, or Cu vapor. For example, asuitable laser may be a NdYAG Pulsed Laser output of 1 joule/pulse, witha pulse width of 1-5 msec, and a pulse strength of 1000-2000 watts. Thepower required to generate the required heat is thought typically to be1-10 watts or more of average power. (A 1 msec, 1000 watt pulse everysecond would be equivalent to 1 watt of CW operation.) In certainembodiments of the present invention, according to FIG. 9, for example,configured with a carbon dioxide laser, a laser beam may be provided tolaser housing 138 directly from a laser beam generator (not depicted)without the use of optic fiber.

In addition to ribbon bonding loops between bond pads as describedabove, this bonding system can also be used to make solder or brazeconnections between ribbons and substrates. The solder or braze materialmay already be on the ribbon or substrate or may be an alloy formedduring the joining operation. High or low temperature solders and brazescan be used even on temperature sensitive substrates. Because of thevery localized heat imparted by laser beam 140, damage to underlying andadjacent materials is avoided.

The bonder could be made as a fully automatic, semi-automatic or manualmachine. The difference among these applications would lie primarily inthe use of programmability and pattern recognition features, as arecurrently available in ultrasonic bonding machines of the prior art. Thetable, ribbon feed and pattern recognition system of an automaticultrasonic ribbon bonder may be utilized to implement an embodiment ofthe subject invention. Accordingly, the present invention may beimplemented, in certain embodiments, by modification of existing bondingtables and other bonding equipment utilizing automation techniques ofcircuit manufacture, such as pattern-matching and machine visiontechnologies. For example, bonding head parts and assemblies areavailable from various manufacturers, such as Orthodyne Electronics, ofIrvine, Calif.; MicroJoin, Inc. (formerly Hughes/Palomar Technologies)of Poway, Calif.; Verity Instruments, Inc., of Carrollton, Tex.; Kulicke& Soffa Industries, Inc. of Willow Grove, Pa.; and F & K Delvotec ofFoothill Ranch, Calif. For example, Orthodyne Model 360S Small WireBonder or the Delvotec 6400 bonder are thought to provide suitable baseunits that may provide basic bonding automation functions such as thosethat may be utilized in accordance with the present invention.

In a preferred embodiment of the subject invention, the laser bondingprocess as described is automated. For example, a device may bepresented to the bonder by manual placement on a work holder orautomatically by a conveyor system. The position of the device may bedetermined by pattern recognition, as is known in the art. Preferably,pattern recognition systems and motion algorithms automaticallycompensate for variations in positions of the bond sites within thevarious assemblies in order to provide automation of the bondingprocess. It is believed that, in accordance with a preferred embodimentof the subject invention, throughput of at least one ribbon connectionper second may be achieved; equating with two laser firings per second.

In one embodiment of the present invention, the above method may be usedto bond a nickel-clad copper ribbon 0.002 inches×0.015 inches (2×15mils). In alternate embodiments of the subject method, ribbons of Pt, Ni205, Ni 270, and Al 6061 may be laser bonded using the above method. Inany embodiment of the invention, materials must adsorb sufficient lightfrom the laser such that their heat is increased above solidus. Certainhighly reflective materials may not absorb sufficient laser light toeffect sufficient heat rise.

It will be appreciated that the present invention, in variousembodiments, can be used for soldering, brazing and welding a widerrange of materials than possible with resistance welding, ultrasonicwelding or soldering alone. It is anticipated that bonds and connectionseffected according to the instant invention, particularly when effectedas security welds, will be highly robust and reliable, and will besignificantly more robust and offer a reduced error rate in comparisonto existing automated bonding methods.

It is believed that a wide variety of connection materials may besuitably bonded by the present invention in one of various embodiments.For example, copper, gold or other ribbon materials could be used. It isanticipated that this will prove particularly useful for non-rigidstructures, which are prone to vibration during ultrasonic processes ofthe prior art. In contrast to bonding systems of the prior art, thepresent invention requires no special fixation or holding methods orapparatus to hold components in order to eliminate vibration duringbonding, because there is no ultrasonic energy to be imparted to theparts to be joined.

In further contrast to existing methods of conductive connectionbonding, the present invention provides a method of bonding with limitedequipment wear and deterioration. For example, ultrasonic bondingequipment, especially for ultrasonic frequencies exceeding 100 KHz, issubject to bond head deterioration resulting from the extreme vibrationand frictional forces endured by the bond head. Similarly, resistancewelding equipment is subject to electrode wear or oxidation.Furthermore, unlike resistance welding, the present invention admits ofbonding low resistance metals such as copper and gold. Gold or copperribbons are commercially available and preferred for their lowresistance, good looping characteristics and corrosion resistance.However, other materials such as nickel and silver could also be welded.

While a preferred embodiment of the present invention has beendescribed, it will be appreciated by those skilled in the art thatvarious changes, adaptations and modifications may be made thereinwithout departing from the spirit of the invention and the scope of theappended claims.

What is claimed is:
 1. A method for bonding of ribbon to a firstsubstrate, comprising: disposing a segment of an elongatedlaser-weldable ribbon through a threading aperture of a bond head,wherein said threading aperture comprises structure constraining lateralmovement of the elongated laser-weldable ribbon, and wherein a portionof said elongated laser-weldable ribbon passes between an articulatedbond head and a first substrate, and wherein an aperture formed in thearticulated bond head is adapted to permit passage of a laser beamtherethrough; aiming a laser beam source so that the laser beam passesthrough the aperture and impinges upon a portion of the elongatedlaser-weldable ribbon; and firing the laser beam source for apredetermined time sufficient to form a first weld nugget at a firstbond site, the first weld nugget consisting of material from both theelongated laser-weldable ribbon and the first substrate.
 2. A methodaccording to claim 1, wherein the laser beam is focused adjacent thebond site.
 3. A method according to claim 1, wherein the elongatedlaser-weldable ribbon is an electrically conductive material having ageometric cross-sectional shape, a round cross-sectional shape, anelongated cross-sectional shape or a substantially rectangularcross-sectional shape.
 4. A method according to claim 1, wherein theelongated laser-weldable ribbon comprises at least a one of the group:nickel-clad copper, platinum, nickel 505, nickel 270, aluminum
 6061. 5.The method of claim 1, wherein the laser beam source operates either ina pulsed mode or in a continuous wave mode.
 6. A method according toclaim 1, wherein said laser beam source is a neodymium yttrium aluminumgarnate laser source, an argon-ion laser source, a carbon dioxide lasersource, or a copper vapor laser source.
 7. A method according to claim1, further comprising the steps of: moving the articulated bond headrelative to a work piece to position the articulated bond head adjacentto a second bond site disposed on a second substrate while playing outthe elongated laser-weldable ribbon from a ribbon spool; disposing theelongated laser-weldable ribbon between the articulated bond head andthe second substrate; aiming the laser beam source so that the laserbeam passes through the aperture and impinges upon the elongatedlaser-weldable ribbon; and firing the laser beam source for apredetermined time sufficient to form a second weld nugget at the secondbond site, the second weld nugget consisting of material from both theelongated laser-weldable ribbon and the second substrate.
 8. The methodof claim 7, further comprising the additional step of severing theelongated laser-weldable ribbon adjacent the second bond site.
 9. Themethod of claim 8, wherein the elongated laser-weldable ribbon issevered by the articulated bond head to form a wedge bond.
 10. Themethod of claim 1, further comprising the steps of: moving thearticulated bond head horizontally, vertically or radially with respectto a location proximate to the first bond site, and firing the laser foran additional predetermined time sufficient to form a security weld. 11.An apparatus for bonding a section of laser-weldable ribbon to at leastone substrate pad, comprising: a laser beam generator; a bond footmoveable with respect to the laser beam generator and having an apertureformed through a portion of said bond foot for passage of a laser beamemitted from the laser beam generator therethrough; aiming means fordirecting the laser beam to a bond site; and a ribbon-pathway means forsmoothly deploying a length of elongated laser-weldable ribbon materialfrom a ribbon source to a position adjacent the aperture.
 12. Anapparatus according to claim 11, wherein the ribbon source comprises aportion of elongated laser-weldable ribbon disposed upon a reciprocatingribbon-storing spool; and further comprising means for preventing theuncontrolled release of the portion of ribbon from the ribbon-storingspool.
 13. An apparatus according to claim 12, wherein the preventingmeans comprises a mechanical clamp.
 14. An apparatus according to claim13, wherein the ribbon-pathway means further comprises structuredisposed on opposing sides of a ribbon pathway to restrain the elongatedlaser-weldable ribbon from moving laterally relative to a majorlongitudinal axis of said elongated laser-weldable ribbon.
 15. Anapparatus according to claim 11, further comprising magnification meansadapted for visually magnifying a region, said region including at leasta portion of the bond site.
 16. An apparatus according to claim 15,wherein the magnification means further comprises at least onemagnifying lens coupled to a part of the apparatus and disposed adjacenta mirror directing the laser beam to the bond site and wherein themagnifying lens and the mirror have different optical paths to the bondsite.
 17. An apparatus according to claim 11, wherein the aiming meansfurther comprises an optic fiber coupled to the laser beam generator ata proximal end and having a portion disposed in the aperture of the bondfoot and wherein a distal end terminates adjacent the bond site.
 18. Theapparatus of claim 17, wherein the optic fiber adjacent the bond head isfirmly coupled to a portion of the bond head.
 19. An apparatus accordingto claim 11, wherein the laser beam generator is a one of the following:a neodymium yttrium aluminum garnate, an argon-ion, a carbon dioxide, ora copper vapor laser beam generator.
 20. An apparatus according to claim11, further comprising a means for articulating the bond head relativeto the bond site.
 21. A bonding apparatus for connecting a segment of aribbon to at least one substrate pad member that is coupled to astationary, substantially unconstrained work piece to form a bondconnection at a bond site, the apparatus comprising: a stationary laserbeam source; a pedestal moveable with respect to the stationary laserbeam source and including a bond foot with an aperture for admitting alaser beam emanating from the laser beam source and a ribbon alignmentmeans for positioning a ribbon between the bond site and the bond footsuch that a part of the ribbon aligned with said aperture; and an aimingstructure to direct the laser beam at the bond site; wherein said ribbonalignment means further comprises structure disposed on at least oneside of a predetermined ribbon pathway to constrain a portion of ribbonfrom twisting or slipping out registration of said predetermined ribbonpathway.